Method for producing a motor vehicle component and motor vehicle component

ABSTRACT

In a method for producing a motor vehicle component and a motor vehicle component produced according to the invention a steel sheet with a stacking fault energy between 10 and 40 mJ/m 2  and a manganese content between 10 and 30% is provided, which is prone to twin formation at room temperature and has at least regions with a predominantly austenitic microstructure. Regions of this steel sheet are first temperature treated to a temperature between +30° C. and −250° C. and subsequently cold formed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application, Serial No. 10 2012 111 959.7, filed Dec. 7, 2012, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a motor vehicle component and to a motor vehicle component.

The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.

From the state of the art it is known to produce self-supporting motor vehicle bodies from metallic components. For this, motor vehicle columns, sills, roof pillars but also longitudinal or transverse members are initially produced and then joined to subassemblies and subsequently to the complete motor vehicle body.

To meet the demand for a motor vehicle that can be operated economically, in recent years materials alternative to that of steel were used. For example the aforementioned motor vehicle bodies were produced from lightweight metal in particular from aluminum which, however, are associated with high production costs due to expensive raw materials and complex processing methods.

Further, the demands on the crash-performance of a motor vehicle are ever increasing while at the same time costs are to be reduced. Thus, such a motor vehicle and with this also a motor vehicle body or other motor vehicle structural components or vehicle body parts have to be particularly lightweight, have to have a high crash safety and have to be capable of being produced cost effectively.

For this, high strength and ultra high strength steels were developed in recent years which compared to lightweight steels or fiber composite materials can be produced cost effectively and at the same time have a particularly high stiffness and with this crash safety at low own weight.

For example, the hot forming and press hardening technology makes it possible to initially heat hardenable steels to above austenizing temperature and to then hot form and harden the steels. Associated therewith is a high strength, however, at limited ductility properties of the component. Depending on the circumstances, complex heat treatments are required in order to set a ductility in sub-regions in a targeted manner.

An alternative to the hot formed and press hardened steels are the so called TWIP steels in which a strong mechanical twin formation occurs in austenitic steel at plastic deformation. The process already starts at low stress and hardens the steel at high elongation at break. The twin formation acts hereby for the dislocation movement in the manner of grain boundaries in the microstructure of the material and thus acts as resistance that counteracts a further shape change. The stretch induced twin formation results in a higher ductility. The martensitic regions result at the same time in a high strength.

It would be desirable and advantageous to obviate prior art shortcomings and to provide an improved production method and a motor vehicle component that can be produced efficiently, has a particularly low own weight and a high strength while at the same time having a high ductility.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method according to the invention for producing a metallic motor vehicle component includes the steps of:

-   -   providing a steel sheet with a manganese content from 10 to 30%         and a stacking fault energy from 5 to 50 mJ/m², in particular 10         to 40 mJ/m², wherein the material of the steel sheet at room         temperature is prone to twin formation and has a least regions         which have a predominant austenitic microstructure. heat         treating at least regions of the steel sheet to a cold forming         temperature between +30° C. and 250° C.,     -   forming the steel sheet to the motor vehicle component at         essentially the cold forming temperature, wherein a martensite         formation is induced at least in regions, and     -   retrieving the steel sheet component,

According to the invention a metallic component in particular made of a high manganese containing steel material particularly, preferably made of a TWIP steel, is provided having a predominantly austenitic microstructure. The component is then further cooled to a cold forming temperature which lies essentially below the room temperature, in particular between +30° C. and −250° C., particularly preferably between +25° C. and −200° C. Subsequent to the cooling, the cooled steel sheet is cold formed to the desired steel sheet component when the cold forming temperature is reached. The cold formed component is then retrieved from the forming tool.

During the cold forming, the plastic deformation results in a transformation of the essentially austenitic microstructure into an at least partial martensitic microstructure, preferably a complete martensitic microstructure. This results in a corresponding increase of the strength properties of the component. This is a deformation induced marteniste formation. At the same time this also leads _(t)o an increase of hardness and formability in the case of plastic stress during production of the motor vehicle component. A silicone content causes a solid solution hardening which increases the yield strength of the component, As a result of the plastic shape change, the metastable, carbon-rich austenite is transformed, induced by deformation, into martensite which causes hardening of at least regions of the motor vehicle component by twin formation owing to the TWIP effect.

At the same time according to the invention the TRIP effect is also utilized which causes a particular martensite formation at forming. Here, in particular the TRIP effect occurs at deformation-induced martensite formation. The TRIP effect at the same time increases the hardness and the formability in case of plastic stress. The TRIP effect is particularly characterized in that as soon as the plastic range is reached during forming, the metastable, carbon-rich austenite is transformed into martensite induced by deformation. This results in a targeted hardening of the steel during plastic deformation.

According to another advantageous feature _(o)f the invention, a steel alloy is used for producing the motor vehicle component according to the invention which has the following alloy components expressed in weight percent:

Carbon (C) max. 2% Manganese (Mn) 10 to 30% Silicone (Si) max. 6% Aluminum (Al) max 8% Niobium (Nb) max. 1% Vanadium (V) max 1% Titanium (Ti) max 1%

Remainder iron (Fe) and smelting related contaminations.

Targeted selection of the alloy components adjusted to the cold forming temperature at which the component is formed, allows targeted adjustment of strength properties of at least regions of the motor vehicle.

In particular, cooling of the steel sheet results in lowering of the stacking fault energy so that the twin formation resulting from the TWIP effect is lowered to a negligible degree. At the same time, however, the transformation of the metastable austenite into martensite is increased, which in turn increases the strength properties of the component. In particular, a good strength property is achieved when the set degree of deformation of the component corresponds to at least the uniform elongation of the used alloy material.

Depending on the used alloy composition and the cold forming temperature of the steel sheet adjusted thereto, the goal is to transform as much austenitic microstructure into martensite by the plastic deformation as possible.

A further control parameter is the degree of deformation itself. The higher the deformation degree, the more pronounced is the transformation of austenite into martensite.

Particularly advantageous strength properties have resulted when a steel sheet with a stacking fault energy between 5 and 50 mJ/m², in particular from 20 to 40 mJ/m² is used as starting material and subsequently cooled to the cold forming temperature, wherein in particular the cooling results in targeted reduction of the stacking fault energy within the steel sheet to a value between 15 and 20 mJ/m².

The cooling itself can occur using different cooling media, in particular liquid nitrogen. By means of the liquid nitrogen it is in particular cooled to a cold forming temperature between +25° C. and −200° C., particularly preferably to a cooling temperature which is in the range form +25° C. to −197° C. Within the framework of the invention this disclosure is to be understood that cold forming temperature relates to any temperature in the last mentioned interval, i.e., in the range from about +25° C. to −197° C. it is thus possible to cool the component for example to a cold forming temperature of −180° C. but also of −100° C. or any desired value from +25° C. to −197° C. In particular the steel sheet is pre-stretched at cold forming temperature.

According to another advantageous feature of the invention, at least regions of the component are cooled or temperature treated. This makes it possible to set the desired strength properties in a targeted manner only in the regions of the component. Due to fast cooling times, the heat conduction in the steel sheet itself for example from a cooled region to a not cooled region is negligible in the framework of the invention.

The cooling itself can in particular occur in a cooling station, wherein the cooled steel sheet after the cooling is transferred into a forming tool, wherein the forming tool is in particular cooled itself. The forming then occurs in the forming tool at essentially the cold forming temperature. A slight heating during the transfer and/or in the forming tool is again negligible with in the framework of the invention.

In an alternative it is possible that the steel sheet itself is cooled in the forming tool to the cold forming temperature and subsequent thereto is directly cold formed.

According to another advantageous feature of the invention, at least regions of the steel sheet can be preformed, in particular the preforming can occur at or above room temperature. Within the framework of the invention the preforming and thus in particular in the range between 0° C. and +50° C., particularly preferably at +20° C. to +30° C. Subsequent thereto the preformed steel sheet is then again cooled to the cold forming temperature and then cold formed.

According to another advantageous feature of the invention, the pre-formed regions can be directly formed to a final dimension, wherein the cold forming occurs subsequently in regions that are different from the pre-formed regions. Thus only the regions that are not pre-formed are temperature treated or cooled and then cold formed. As a result the pre-formed regions have a lower strength than the cold formed regions.

According to another advantageous feature of the invention, regions may also only be partially pre-formed in a targeted manner, wherein the pre-forming is carried out for example as pre-stretching in particular at cold forming temperature. The pre-stretching itself can also be carried out partially. The deformation degree in the pre-stretching is in particular 10 to 90% of the final dimension. Within the framework of the invention it is then possible to again cool and cold form the pre-stretched or pre-formed regions, wherein the degree of the martensite formation can be set in a targeted manner by the degree of the pre-forming or pre-stretching.

The present invention also includes a motor vehicle component which is produced according to a method with at least one of the aforementioned features, wherein the motor vehicle component is made of a TWIP steel alloy and is characterized according to the invention in that at least regions of the component have an essentially martensitic microstructure. The production method according to the invention thus allows reducing the TWIP effect i.e., the mechanical twin formation, and at the same time generating a higher martensite proportion in the regions which were formed at cold forming temperature.

In particular, the motor vehicle component is made of a steel alloy preferably with high manganese content and has the following alloy components expressed in weight percent

Carbon (C) max. 2% Manganese (Mn) 10 to 30% Silicone (Si) max. 6% Aluminum (Al) max 8% Niobium (Nb) max. 1% Vanadium (V) max 1% Titanium (Ti) max 1% Remainder iron and smelting related impurities.

In particular this steel alloy is a TWIP steel alloy which depending on the desired strength properties is selected to have the corresponding percentage and presence of the individual alloy elements.

As a result of the production method according to the invention, the motor vehicle component has in the martenistic regions preferably an elongation at break of R_(p0.2) between 500 and 1500 MPa, in particular between 700 and 1300 MPa and particularly preferably between 750 and 1000 MPa. In the case of a component produced with different regions, the remaining regions then have an elongation at break between 200 and 800 MPa, in particular between 300 and 500 MPa.

Further preferably, the motor vehicle component has a tensile strength Rm between 500 and 1800 MPa, in particular between 800 and 1700 MPa and particularly preferably between 1000 and 1650 MPa. In the components in which only regions are cooled and formed, the remaining regions then have a tensile strength from 500 to 1500 MPa, in particular from 800 to 1200 MPa and particularly preferably from 850 to 1100 MPa.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 shows a stress-strain diagram of a steel produced according to the invention at three different temperatures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

Turning now to the drawing, and in particular to FIG. 1, there is shown a stress-strain diagram of a steel formed according to the invention wherein three different cold forming temperatures were selected. It can be seen that the lower the cold forming temperature was selected the more the tensile strength increases. Thus, the steel of the curve 1 was formed at room temperature, i.e., essentially at 20° C. The material in the curve 2 was formed at −110,15° C. and has a significantly higher tensile strength relative to the forming at room temperature. The component according to the curve 3 was formed at −196,15° C. and has still a significantly increased tensile strength.

While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: 

What is claimed is:
 1. A method for producing a metallic motor vehicle component, comprising: providing a steel sheet with a manganese content from 10 to 30% and a stacking fault energy from 5 to 50 mJ/m², in particular 10 to 40 mJ/m² wherein the material of the steel sheet at room temperature is prone to twin formation; temperature treating at least regions of the steel sheet to a cold forming temperature between +30° C. and −250° C.; forming the steel sheet to the motor vehicle component at substantially the cold forming temperature, wherein a martensite formation is induced at least in regions of the steel sheet component; and retrieving the motor vehicle component from the forming tool.
 2. The method of claim 1, wherein the steel sheet is made of a steel alloy comprising the following alloy components in weight %: Carbon (C) max. 2% Manganese (Mn) 10 to 30% Silicone (Si) max. 6% Aluminum (Al) max 8% Niobium (Nb) max. 1% Vanadium (V) max 1% Titanium (Ti) max 1%

Remainder iron (Fe) and smelting related impurities.
 3. The method of claim 1, wherein the steel sheet is made of a TWIP steel.
 4. The method of claim 1, wherein the predominantly austenitic microstructure is transformed into a martensitic microstructure by the forming step.
 5. The method of claim 1, wherein the steel sheet has a stacking fault energy of 20 to 40 mJ/m², and wherein the stacking fault energy is reduced to 15 to 20mJ/m² by the temperature treating of the steel sheet to the cold forming temperature.
 6. The method of claim 1, wherein liquid nitrogen is used for the temperature treatment.
 7. The method of claim 1, wherein the cold forming temperature is between +25° C. and −200° C., in particular in the range from +25° C. and −197° C.
 8. The method of claim 1, further comprising pre-stretching the steel sheet at cold forming temperature.
 9. The method of claim 1, wherein the temperature treatment of the steel sheet occurs in a cooling station and the cooled steel sheet is transferred into a forming tool.
 10. The method of claim 9, wherein the forming tool is cooled.
 11. The method of claim 10, wherein the steel sheet is cooled to the cold forming temperature in the forming tool.
 12. The method of claim 1, wherein at least a region of the steel sheet is pre-formed.
 13. The method according of claim 13, wherein the pre-forming occurs at or above room temperature.
 14. The method according of claim 13, wherein the preformed regions are formed to final dimension, and wherein the cold forming occurs subsequently in regions which are different from the pre-formed regions.
 15. A motor vehicle component produced according to the method of claim 1, wherein the motor vehicle component is made of a TWIP steel alloy, and wherein in at least sub-regions of the component have an essentially martensitic microstructure.
 16. The motor vehicle component of claim 15, comprising the following alloy components in weight %: Carbon (C) max. 2% Manganese (Mn) 10 to 30% Silicone (Si) max. 6% Aluminum (Al) max 8% Niobium (Nb) max. 1% Vanadium (V) max 1% Titanium (Ti) max 1%

Remainder iron (Fe) and smelting related impurities.
 17. The Motor vehicle component of claim 15, wherein the martensitic regions have an elongation at break R_(p02) between 500 and 1500 MPa, in particular between 700 and 1300 MPa and particularly preferably between 750 and 1000 MPa. 